Nonaqueous-electrolyte batteries such as lithium secondary batteries are coming to be practically used in extensive applications ranging, for example, from so-called household power sources for portable telephones, notebook personal computers, and so on to driving batteries equipped on vehicles such as automobiles. However, recent nonaqueous-electrolyte batteries are increasingly required to have higher performances, and there is a desire for improvements in battery characteristics.
Electrolytic solutions for use in nonaqueous-electrolyte batteries are usually constituted mainly of an electrolyte and a nonaqueous solvent. As main components of the nonaqueous solvent, for example, cyclic carbonates such as ethylene carbonate and propylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone; and the like is used.
Various investigations are being made on nonaqueous solvents and electrolytes in order to improve the battery characteristics of such nonaqueous-electrolyte batteries, such as load characteristics, cycle characteristics, and storability, or to enhance battery safety during overcharge.
Patent document 1 proposes a technique in which an additive that polymerizes at a battery voltage not lower than the maximum working voltage of a battery is incorporated into an electrolytic solution to thereby increase the internal resistance of the battery and protect the battery. Patent document 2 proposes a technique which by mixing an additive into an electrolyte, which polymerizes and thereby generates gas and pressure, at a battery voltage not lower than the maximum working voltage, thus enable an internal circuit breaker for the purpose of protection against overcharge to work without fail. Aromatic compounds such as biphenyl, thiophene, and furan are disclosed as those additives.
Furthermore, patent document 3 proposes a nonaqueous-electrolyte secondary battery system including: a nonaqueous-electrolyte secondary battery in which phenylcyclohexane has been added to the nonaqueous electrolytic solution in an amount in the range of 0.1-20% by weight in order to inhibit battery characteristics from decreasing when biphenyl or thiophene is used; and a charge control system which detects an increase in battery temperature to break the charging circuit.
On the other hand, patent document 4 proposes a technique in which an organic solvent selected from carbonates having a phenyl group, such as methyl phenyl carbonate, esters, and ethers is used to thereby improve the affinity of the electrolytic solution for carbon electrodes and attain size reduction, performance enhancement, and productivity improvement in lithium ion batteries.
Patent document 5 proposes that a phosphoric acid triester should be added to an electrolytic solution in order to improve cycle characteristics.
Furthermore, patent document 6 proposes an electrolyte characterized by containing a solvent including diphenyl carbonate in an amount in the range of from 0.5% by mass to 10% by mass and further including a cyclic carbonic acid ester of an unsaturated compound in an amount in the range of from 0.5% by mass to 5% by mass, in order to inhibit battery swelling during high-temperature storage while inhibiting the efficiency of initial charge/discharge from decreasing. There is a statement therein to the effect that vinylethylene carbonate is preferred as the cyclic carbonic acid ester of an unsaturated compound.    Patent Document 1: JP-A-9-106835    Patent Document 2: JP-A-9-171840    Patent Document 3: JP-A-2002-50398    Patent Document 4: JP-A-8-293323    Patent Document 5: JP-A-7-114940    Patent Document 6: JP-A-2005-322634